By the looks of it there will be a release or 2.7.x.x for the 85x/95x series of Blackberry's but that's about as good as it gets, development has been limited to the older C++ devices since the release of the J2ME/Java Blackberry's, by the looks of it the Mobitex/DataTAC Blackberry 5790 will come to market for user that require 2Way messaging and paging but with the added Java platform. As for the 5810/5820 I don't think there's enough flash memory on those units to run OS 4.0? If you're looking for a OS 4.0 capable device your most likely going to have to upgrade.

From what I am able to tell, there seems to be a very stripped-down 58XX operating system upgrade of some kind coming up. What version it will be, I don't know, but one of the betas were labelled 4.0 but is a subset of it (i.e. limited browser) It may support fuller PIM like categories, etc. Not sure.